An electrophotographic, or xerographic, image printing system employs an image bearing surface, such as a photoreceptor drum or belt, which is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the image bearing surface is exposed to a light image of an original document being reproduced. Exposure of the charged image bearing surface selectively dissipates the charge thereon in the irradiated areas to record an electrostatic latent image on the image bearing surface corresponding to the image contained within the original document. The location of the electrical charge forming the latent image is usually optically controlled. More specifically, in a digital xerographic system, the formation of the latent image is controlled by a raster output scanning device, usually a laser or LED source.
After the electrostatic latent image is recorded on the image bearing surface, the latent image is developed by bringing a developer material into contact therewith. Generally, the electrostatic latent image is developed with dry developer material comprising carrier granules having toner particles adhering triboelectrically thereto. However, a liquid developer material may be used as well. The toner particles are attracted to the latent image, forming a visible powder image on the image bearing surface. After the electrostatic latent image is developed with the toner particles, the toner powder image is transferred to a media, such as sheets, paper or other substrate sheets, using pressure and heat to fuse the toner image to the media to form a print.
The image printing system generally has two important dimensions: a process (or a slow scan) direction and a cross-process (or a fast scan) direction. The direction in which an image bearing surface moves is referred to as the process (or the slow scan) direction, and the direction perpendicular to the process (or the slow scan) direction is referred to as the cross-process (or the fast scan) direction.
Electrophotographic image printing systems of this type may produce color prints using a plurality of stations. Each station has a charging device for charging the image bearing surface, an exposing device for selectively illuminating the charged portions of the image bearing surface to record an electrostatic latent image thereon, and a developer unit for developing the electrostatic latent image with toner particles. Each developer unit deposits different color toner particles on the respective electrostatic latent image. The images are developed, at least partially in superimposed registration with one another, to form a multi-color toner powder image. The resultant multi-color powder image is subsequently transferred to a media. The transferred multicolor image is then permanently fused to the media forming the color print.
Banding generally refers to periodic defects on an image caused by a one-dimensional density variation in the process (slow scan) direction. An example of this kind of image quality defect, periodic banding, is illustrated in FIG. 1. As shown in FIG. 1, bands exist in columns 1a, 1b, 1c, 1d, 1e, 1f and 1g. Banding in a xerographic engine may be caused by charge non-uniformity on the image bearing surface, variations in a Photo Induced Discharge Curve (PIDC), image bearing surface motion quality variations, and/or image bearing surface “out-of-round” that lead to periodic non-uniformities manifesting in the output print. The PIDC may be defined as a plot of surface potential of the image bearing surface as a function of incident light exposure. For an example of a system and method for generating a PIDC, see U.S. Pat. No. 6,771,912, herein incorporated by reference in its entirety. Image bearing surface motion quality variation may be defined as imperfections in the motion of the image bearing surface causing the instantaneous position of the image bearing surface to be less than ideal. Image bearing surface motion quality variations may be caused by vibration, motion backlash, gear train interactions, mechanical imbalances, friction, among other factors. Image bearing surface out-of-round may be defined as variations in the diameter of the image bearing surface, such as a photoreceptor drum, causing the image bearing surface to not be perfectly round. These problems can exist at build, or through degradation with component age. Costly part replacement has been used in the past to counteract these problems.
Several different methods and systems exist for measuring image quality defects. These methods and systems usually use sensors in the form of densitometers, including Automatic Density Control (ADC) sensors, to measure image quality defects in an output print. Generally, a densitometer measures the degree of darkness for an image. In particular, an ADC sensor may measure the light reflected from the toner image on an intermediate transfer belt, and supplies a voltage value corresponding to the measured amount of light to a controller. The problem with an ADC reading is that sources of noise due to development, first transfer, and retransfer on downstream image bearing surfaces are introduced, therefore decreasing the signal-to-noise ratio (SNR).